1. Field of the Invention
The present invention relates to a thermoreversible recording medium which possesses an excellent property that accumulation of electrostatic charge on the thermoreversible recording medium may be prevented and the curling of the thermoreversible recording medium caused by repeated heating for printing and erasing of the thermoreversible recording medium also may be prevented, and in addition possesses an excellent conveyability which is not affected by repeated use of the thermoreversible recording medium and by a condition of use thereof, and also relates to a thermoreversible recording label, a thermoreversible recording member, an image processing apparatus and a process which employ the thermoreversible recording medium respectively.
2. Description of the Related Art
In recent years, a thermoreversible recording medium (hereinafter, sometimes referred as “reversible thermosensitive recording medium” or “recording medium”) on which a temporary image may be formed and the formed image may be also erased, when the image is not necessary more, attracts much attention. As a representative example of such cording media, a thermoreversible recording medium produced by dispersing a color developer, such as an organic phosphorus compound, aliphatic carboylic acid compound and phenol compound which contain a long-chain aliphatic hydrocarbon group and a coloring agent, such as a leuco dye in a resin composition, is well-known (see Japanese Patent Application Laid-Open (JP-A) Nos. 5-124360 and 6-210954).
Many of such thermoreversible recording media comprise PET film having a magnetic recording layer as a support and are used commercially as a material for mainly a point card. On the other hand, many methods for producing a thermoreversible recording medium are proposed, wherein the thermoreversible recording medium is produced by laminating a multi-layer unit in which a thermoreversible recording layer is disposed on a surface of a thin support and an adhesive layer is disposed on another surface of the support, on various kinds of substrates with applying heat or pressure. A multi-layer unit comprises a thermoreversible recording layer, a thin support and an adhesive layer, wherein a thermoreversible recording layer is disposed on a surface of the support and the adhesive layer is disposed on another reverse surface of the support (see JP-A Nos. 2000-94866, 2000-251042, 2001-63228 and 2002-103654).
However, in these proposed methods, examples of the above-noted substrates included substrates for optical memory, contact type IC, non-contact type IC and magnetic recording and since these substrates were mostly very thick, the size of cards produced by using these substrates was limited and the application purpose of these cards was also limited. In other words, these cards were not suitable for an enter-exit ticket, stickers for containers of frozen foods, industrial products and various medicines, and wide screens indicating various informations for controls of product distribution and production process.
Therefore, for above-noted application purposes, a thermoreversible recording medium having a size of “sheet size” which is larger than card size is necessary to be used. Here, “sheet size” means a size which is larger than card size (54 mm×85 mm).
When the above-noted thermoreversible recording medium is used as a sheet, the size of the recording medium becomes larger than the size of a point card or a card made of a thick substrate. Accordingly, when such a thermoreversible recording medium is conveyed by the printer, the recording medium becomes easily electrostatically charged by the contact of a recording medium with another recording medium or with a conveying roller of the printer and a static charge accumulated on a thermoreversible recording medium becomes larger, because of a larger contacting area of a thermoreversible recording medium with another thermoreversible recording medium or with a conveying roller of the printer. As a result, thermoreversible recording media stick to each other and the thermoreversible recording medium may be difficultly conveyed by the printer. On the other hand, a thermoreversible recording medium having a large size poses a problem that since the thermoreversible recording medium is shrunk by repeating the printing and erasing by heating, the curling is caused on the thermoreversible recording medium and a large curling may cause a defect in conveyance of the thermoreversible recording medium.
There is reported a thermoreversible recording medium in which an anti-static effect thereof is improved for solving the above-noted problem. For example, in JP-A No. 11-254822, there is proposed a thermoreversible recording medium having a surface resistance of 1×1013 ohm/square or less (measured at 20° C. and under a relative humidity of 65%) and a surface static friction coefficient of 0.65 or less. However, in this proposal, the thermoreversible recording medium has a lower surface resistance measured under a low humidity and particularly with respect to a thermoreversible recording medium having a surface resistance of 1×1011 ohm/square or less, disadvantage is caused in that since the static charge cannot be satisfactorily removed from the thermoreversible recording medium under a low humidity and the thermoreversible recording medium is charged by repeating the printing and erasing under a low humidity, thermoreversible recording media stick to each other in the printer and then, a defect in conveyance of the thermoreversible recording medium is caused. There is posed also a problem that the curling on the thermoreversible recording medium becomes larger by repeating the use of the thermoreversible recording medium and it results also in a defect in conveyance of the thermoreversible recording medium.
In JP-A No.10-250239, there is proposed a thermoreversible recording medium comprising conductive particles having a shortest diameter of 1 μm or less. In this proposal, a less amount of dust attaches to the thermoreversible recording medium, however there is neither disclosed nor suggested a description with respect to a surface form of the thermoreversible recording medium and when thermoreversible recording media having a surface which is mentioned in the proposal are piled in the printer, they may be difficultly conveyed by a paper feeding roll in the printer. As a result, sheets of thermoreversible recording media cannot be separated into an individual sheet and then, the conveyablity of the thermoreversible recording medium is impaired in the printer. In addition, the proposed thermoreversible recording medium poses a problem that during repeating the printing and erasing of the thermoreversible recording medium, the curling is caused by heating for the printing and erasing and the conveyablity of the thermoreversible recording medium is impaired in the printer.
Further, in JP-A No.11-91243, there is proposed a thermoreversible recording medium comprising at least one layer comprised of particles of a conductive metal oxide semi-conductor, wherein the particle is a conductive pigment coated with tin oxide. However, it is the same as mentioned in JP-A No.10-250239 above that there is no description with respect to a surface form of the thermoreversible recording medium in the proposal and when thermoreversible recording media having a surface which is mentioned in the proposal are piled in the printer, they may be difficultly conveyed by a paper feeding roll in the printer. As a result, sheets of thermoreversible recording media may not be separated into an individual sheet and then, the conveyablity of the thermoreversible recording medium is impaired in the printer. In addition, the proposed thermoreversible recording medium poses a problem that during repeating the printing and erasing of the thermoreversible recording medium, the curling is caused by heating for the printing and erasing and the conveyablity of the thermoreversible recording medium is impaired in the printer.
In other fields, as a method for imparting an anti-static function to the thermoreversible recording medium, for example, a heat transfer receiving sheet comprising a conductive needle-like crystal is proposed (see JP-A No.11-78255). However, when the method of this proposal is applied to the thermoreversible recording medium, a satisfactory anti-static function of the thermoreversible recording medium cannot be obtained and there is reported no example for forming an anti-static layer on the most outer surface of the thermoreversible recording medium. In this case, the conveyablity of the thermoreversible recording medium is also unsatisfactory. Moreover, in this proposal, during repeating the printing and erasing, disadvantage is caused in that thermoreversible recording media stick to each other and multi feeding of the recording media is caused. Further, in this proposal, the curling of the thermoreversible recording medium cannot be satisfactorily prevented and during repeating the printing and erasing by heating, the curling becomes larger. As a result, there is posed a problem that a defect in the conveyance is caused.
For preventing the curling, there is proposed a thermoreversible recording medium comprising a protective layer (the surface) and a back layer (the reverse surface) (the both layers are made of a ultraviolet-curable resin), wherein a kinetic coefficient of friction between the protective layer and the back layer is 0.3 or more and a kinetic coefficient of friction between 2 protective layers is 0.3 or less (see JP-A No.8-187941). By this proposal, the curling can be effectively prevented; however, the thermoreversible recording medium of this proposal is charged during repeating the printing and erasing and thermoreversible recording media stick to each other, thereby resulting in a defect in the conveyance. Further, during repeating the printing and erasing, to the thermoreversible recording medium heat and pressure are applied by a thermal head and heat is applied by an erasing unit; accordingly the surface property of the thermoreversible recording medium is so changed that a defect in conveyance thereof may be induced. In addition, when thermoreversible recording media are set into the printer in such a wrong setting order that the reverse surface of a thermoreversible recording medium faces to the reverse surface of another thermoreversible recording medium, a kinetic coefficient of friction between a surface and another surface differs from a kinetic coefficient of friction between a reverse surface and another reverse surface and as a result, disadvantage is caused in that a defect in conveyance of the thermoreversible recording medium may be induced.
As noted above, there are a method for preventing the electrostatic charge and a method for preventing the curling individually; however a thermoreversible recording medium which possesses not only such an excellent property that both the electrostatic charge and the curling can be prevented, but also an excellent conveyability which is not affected by repeating the use of the thermoreversible recording medium and by an using condition thereof, and a related technique thereto have not been attained yet.